One way to use unit testing libraries effectively is to use them to give yourself a safe playground for learning other libraries or features of the language.
Say you’re working in the .NET space, and you are trying to learn AutoMapper. One of the easiest ways to try out that library is to spin up a simple unit test project, and test your way into understanding the new framework. Start with simple examples from the docs and assert the values you expect. Try out different functions! Be thorough, and try it all out in the safety of a unit testing project that will not negatively impact your current work.
This has a handy benefit of getting you muscle memory using the library as well.
You can do this for learning programming languages as well. Getting unit tests up and running is a great step in learning a new language, and can be inserted right after the ‘hello world’ step!
Even if you are not a TDD fan, consider starting from scratch with unit testing. Happy coding!